1. Field of the Invention
This invention relates to liquid level sensors and more particularly, to a liquid level sensor device adapted for use in a storage tank having a hazardous and explosive environment.
2. Background
Many industrial fluids are stored in large tanks prior to their distribution for use. It is often necessary to know the fill status of the fluids in the tanks, and for this purpose, liquid level sensors inserted into the tanks are used.
The presently available liquid level sensor devices for industrial, fluid storage utilize electrical assemblies that are contained in one or more sealed tubes, and have a variety of external float mechanisms to activate switches on the electrical assemblies. The status of these switches and therefore, the liquid level, is communicated by wire from the top of the sensor device to any desired monitoring location.
A typical use for these sensors is in the oil tanks and water tanks located near an oil well head. The sensors measure the levels of oil or water in the tanks deposited by the pump at the well head, and transmit the liquid level data to a local or remote monitoring unit.
While the presently available liquid level sensor devices generally perform adequately, they are known to have serious disadvantages and potential problem areas as a consequence of their physical and electronic data system mechanizations.
Since a sensor device usually is located in a turbulent environment inside a liquid storage tank, liquid movement in the vessel impinges on the device, creating substantial flexing of the device tube and the electrical assemblies inside the tube. In the prior art devices, the electrical assemblies on which are mounted sense switches, are generally arranged and joined end to end, with wire connectors and often switches connected across the assembly board ends. After a time, breakage of internal electrical components is observed near or at assembly joints. The device then has to be repaired or replaced by another, which is usually an expensive, time consuming procedure.
Many electrical conductive traces are typically used in the prior art devices to connect and address the switches on the electrical assemblies. As an example, in U.S. Pat. No. 4,976,146, the liquid level sensor utilizes 31 traces to run from the bottom of the unit to its top assembly. This large number of traces requires large connectors and greatly reduces the reliability of the device, increasing connection/connector complexity and the probability of electrical failures.
An additional problem area concerns detection of failed sensor switches inside the device. As yet, none of the prior art available liquid level sensor devices is known to incorporate a satisfactory method of determining which sensor switches have failed. The accuracy and reliability of the liquid level detection is then compromised.
Thus, there exists a need for a liquid level sensor device that can withstand a turbulent liquid environment without internal breakage and has high measurement accuracy and reliability of operation.